1. Field of the Invention
The invention is a method for synthesizing films of zeolites which are bonded to substrates and the structures made by this method. The structures may be used for separations, for the sorption of hydrocarbons, for catalysis, and for Selective Catalytic Reduction (SCR) of NO.sub.x.
2. Description of the Related Art
Synthetic zeolites have been used as adsorptive separation agents for gases and liquids, and as catalysts. Generally, zeolites have been synthesized to yield a powder form of the crystalline material which can be formed or extruded into granules, beads or pellets, often incorporated with a binder such as clay or alumina.
Zeolites both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. The zeolite have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-5 (U.S. Pat. No. 3,702,886), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-20 (U.S. Pat. No. 3,972,983), ZSM-35 (U.S. Pat. No. 4,016,245), ZSM-38 (U.S. Pat. No. 4,046,859), and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5 and up to infinity. U.S. Pat. No. 3,941,871 (Re. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added aluminum in the synthesis mixture and exhibiting the X-ray diffraction pattern characteristics of ZSM-5 type zeolites. U.S. Pat. Nos. 4,061,724; 4,073,865 and 4,104,294 describe crystalline silicates or organosilicates of varying alumina and metal content.
Although zeolites include materials containing silicon and aluminum, it is recognized that the silicon and aluminum atoms may be replaced in whole or in part with other elements. More particularly, Ge is an art recognized substitute for Si and B, Cr, Fe and Ga are art recognized replacements for Al. Accordingly, the term zeolite as used herein shall connote not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and/or aluminum.
Monolithic substrates such as wash coated ceramics, described for example in U.S. Pat. No. 4,771,029, and extruded catalytic monoliths, described for example by Lachman et al., "Extruded Monolithic Catalyst Supports", Symposium on Catalyst Supports: Chemistry, Forming and Characteristics, American Chemical Society, New York City Meeting, August 1991, pp 535-543, have been described as useful in emissions control.
U.S. Pat. No. 4,800,187 describes a method for crystallizing a zeolite on the surface of a ceramic monolith containing silica with a crystallization mixture containing a ratio of H.sub.2 O/SiO.sub.2 of 16-20 to one and a ratio of SiO.sub.2 /Al.sub.2 O.sub.3 of one to 0.0-0.4 for the synthesis of ZSM-5. Different ratios are described for large pore zeolites X and Y.
However, when well known zeolite synthesis techniques are used in an attempt to obtain crystallization of a zeolite film adherent to a substrate, there is a strong tendency for homogeneous crystallization to occur. That is, the crystals form in the crystallization solution and settle to the bottom to yield a powder form rather than bonding to the surface to form a strongly adherent film.
It is therefore an object of the invention to provide a method for synthesizing a zeolite film of continuously intergrown zeolite crystals strongly bonded to the surface of a substrate or monolith.
It is another object of the invention to provide a structure synthesized by this method, the structure having numerous applications for sorption, hydrocarbon conversions, and catalytic conversion of NO.sub.x.